A gas turbine engine is a flow machine in which a pressurized gas expands. The gas turbine includes a turbine or expander, a compressor connected upstream of the turbine, and a combustion chamber between the compressor and turbine. Expanding gas produced in the combustion chamber drives blades of the turbine which provides power for the compressor and other engine output. The compression of air by way of the blading of one or more compressor stages, subsequently mixes the compressed air in the combustion chamber with a gaseous or liquid fuel, where the mixture is ignited by an igniter to initiate combustion. The combustion results in a hot gas (mixture composed of combustion gas products and residual components of air) which expands in the following turbine section, with thermal energy being converted into mechanical energy in the process to drive an axial shaft. The shaft is connected to and drives the compressor. The shaft also drives a generator, a propeller or other rotating loads. In the case of a jet power plant, the thermal energy also accelerates a hot gas exhaust stream, which generates the jet thrust.
The gas turbine engine is designed to operate within certain ranges of pressure, velocity and temperatures of both the air and hot gas combustion products that vary with location through the engine. Optimal performance is achieved within very narrow ranges. Thus, to validate the design or to ensure that the gas turbine engine is operating within specified ranges or to make adjustments to attain the optimal performance, it is desirable to know the actual distribution of temperature, pressure and velocity during operation. Determining such distributions is challenging, at least in part, because the pressures and temperature can become very great.
Current approaches to monitoring the distribution of pressure, temperature and velocity in a gas turbine engine include some intrusive probes that project into the gas flows, including probes mounted on vanes (e.g., Kielhead probes) to obtain some profiles of velocity and temperature. Optical instruments have been used, but the characteristics of the optical devices can degrade at the extreme temperatures in at least portions of the turbine engine. Non-intrusive acoustic approaches have been implemented and promise to avoid deficiencies in other approaches.